Gunsmoke (1955)

Season 1: Episode 3

Hank Worth, son of wealthy rancher Jake Worth, is kidnapped by three men and held for $20,000 ransom. They shoot him in the back for trying to escape. They call on Doc but Hank dies. The killers decide to let Doc go because he is the only doctor within 400 miles and they fear they might need his services someday. Before sparing his life and freeing him, they force the reluctant Doc to give his word of honor that he will never reveal their identities.